Hi anoukaimee and welcome to The Mattress Underground 
I’ve been looking into DIY’ing a hybrid mattress with springs or true HD foam base and/or transition and latex comfort layer, and came across the Foam Factory. I’m on a fixed income, but I have fibromyalgia and I cannot take my old Tuft & Needle anymore; Zinus etc seem like a total crapshoot.
Lately we did have quite a few discussions about fibromyalgia. This condition can be quite complex to mitigate so I am sorry to learn that you have this preexisting condition. Depending on your BMI Post #3 may give you a few insights.
As you probably know, any mattress will feel different to any sleepers using it and this would be based on their Stats (Height, BMI, sleeping position(s), any underlying health issues, and their PPP (Posture and alignment, Pressure relief and Personal preferences) it’s difficult for anyone else to tell what may feel comfortable for you. Good to see that you are keeping in mind the Mattress Durability Guidelines in mind along with the Mattress Specifications It You Need To Know
Foam Factory’s prices entirely undercut what their competitors are charging, so I thought it wise to check here to see if they’re reputable. And it appears that, at least in 2014, they weren’t. Lots of discussion, most of it surrounding dishonest representations of their latex.
But there hasn’t been much discussion of them since then, and I just wanted to confirm that the same holds true today
As you can see this Post and this post (Foam Factory - Poor service, any recommendations for a different supplier?) discuss more recent dealings consumers have had with Foam Factory. You are quite right that in the past TMU gathered quite a few “no-nos” for Foam Factory (aka Foam by Mail and many other names) but it seems that recently they are back on the “radar” with consumer complaints. So yes, I’d be cautious! You are quite right that lower prices than what’s found on the current market can be a red flag but not necessarily. I know of companies that are operating on a very low-profit margin that allows them to sell in quantity. But sometimes “cheap” can have a higher cost than people may realize or hope for.
If you decide to go in that direction it’s good to know that incorrect listing of data specifications for products may get the attention of consumer protection agencies. While in the past some companies may have been able to get away with this, currently any consumer can file a “product misrepresentation” report with such an agency.
I will say that a DIY mattress can be ultimately rewarding, but not for the ‘weak of heart’, and for someone with fibromyalgia may prove a little more difficult as their sensitivity to pressure threshold may be higher than most this it may require more trial and error. So it may be worth looking into something that has good return policies if things don’t go as well as you would hope for.
This wording on Foam Factory’s website would make me pause moving forward.
This product is sold without warranty, expressed or implied. (Unless stated otherwise.) Buyer assumes all responsibility for loss or damage arising from the handling and use of this product, whether done in accordance with directions or not.
It can take much rearranging of layers and potential comfort layer exchanges to ‘dial in’ the correct comfort level, which can take a good bit of time and energy.
Many of the Trusted Members of the site carry hybrid mattresses of latex and foam, like BioSleep Concept, Custom Sleep Technology as well as My Green Mattress, and Nest Bedding. As you are looking at a DIY project, DIY Mattress is also a Trusted Member of the site that specializes in helping consumers find the right layers for their DIY along with a very low price point.
But there hasn’t been much discussion of them since then, and I just wanted to confirm that the same holds true today. And more relevant for me, are they bad source for HD polyfoam, should I choose it as support layer(s) instead of springs for the base level? Their “Lux HQ” data sheet claims it has a 2.8 lb/cu ft density and “25% ILD (lb) 50.” And they have a medium feel foam with the same density that could serve as a good transition layer for one on a budget.
2.8lb/cuft is a good density for PU foams; the base rule of thumb is to avoid any inexpensive low-density foams as they can break down prematurely. They are claiming an ILD of 36 for the HQ foam which would be considered very firm, depending on what your preferences are and where in the mattress you’d use this layer.
I will say that due to the many consumer issues you have seen over the years, the ‘value’ of the low price may be misleading if its’ not what you are comfortable on.
Phoenix