Thanks for choosing DLX and thanks for checking on the support setup.
That is a good idea to replace the metal frame without legs to make sure the mattress has proper support.
As for suggestions, it really depends on how low you want the mattress to sit.
We are not affiliated with them and can’t speak for their service, but customers have reported good experiences with this Ironwood foundation:
It is the least expensive options we’ve found that provides adequate support, and it comes in two heights: 5" and 8".
The next least expensive, but taller, option would be this one from KD Frames.
Again, we are not affiliated with them and can’t speak for them, but it does provide adequate support and is the least expensive options in that class of bed frame. It brings the mattress about 15" off the floor, though I believe they may offer a lower-profile option for an additional cost that brings it closer to 12" off the ground. If you go this route and it does not come with center legs, we would recommend purchasing them from KD Frames.
On the mattress cover, the top quilting does zip off completely, but it is not machine washable. Because of that, we do recommend using a mattress protector especially with young ones in bed.
We don’t have a specific go-to protector we recommend. Any waterproof mattress protector with solid reviews should work well. Many of them are very similar and are often the same product repackaged under different brand names. A few things to keep in mind.
Look for one rated to fit mattresses up to at least 16" thick if you ordered the Premier Hybrid Mattress 14". Styles that go on like a fitted sheet are totally fine. If you’re concerned about it popping off, some options use corner straps or a heavier-duty elastic band can help.
Some waterproof protectors feel stiff out of the package. Washing it a couple of times before use usually helps soften it up. We also suggest looking for fabrics that include a small amount of spandex, Lycra, or elastic, even 1–5%. That little bit of stretch can make a noticeable difference in how it conforms to the mattress and helps avoid the stiff feel you sometimes get with terry cloth styles.
Cooling claims are often overstated. In our experience, any cooling technology effect is minimal and usually fades within the first hour or less. We generally suggest skipping that feature unless you like everything else about the protector or unless you always sleep hot and want to do everything possible to stay cool.
Thanks again and please let us know if you have any other questions.