What is the best product to avoid ulcer on the lower leg, not the heel?

Have you tried a pillow between her legs. I understand kicking off the boot they do get very hot. Try and avoid a pillow under her knees because this may impede the blood flow to her calves and lead to blood clots (DVTs)
Does she actually have an ulcer on her lower leg?
Is she diabetic?
Is she up and able to walk during the day? If not exercise her legs yourself several times a day to keep the blood flowing. Request a PT visit to show you how to do this. There are also alternating pressure wraps but hese are extremely irritating and noisy so she will probably get them off too and I don't blame her.
Can she still understand what is going on?
An alternating pressure mattress could help or just an egg crate. Have you tried those?

Ask the MD about a lateral rotation air mattress, see if your insurance will cover the Rx for one.

Jiminey, A stage one pressure sore is inflamed red skin. Look it up.

Jiminy some of the alternating pressure mattresses can be turned off and go flat allowing you to provide care. Then you just turn it back on

I agree with you that prevention is the best medicine. Check if you can rent such a mattress. Does mom have a hospital bed? You can rent hospital beds with different mattress options.

Thanks Veronica. Mom is a 95 year old and very petite . She has vascular dementia and a diabetic. I've tried a pillow under her legs at night , but I don't know how she kicks it to the side. I was thinking about a waffle boot but I'm going to wat until I speak to her podiatrist next week. I would consider a waffle mattress, but it would be difficult to turn her to change her diapers on it. I have bought so many foot protectors but to no avail She kicks off all the small bots with the Velcro on it. IF it doesn't come off she gets a red rubbing mark from the strap.

In order to get a rotation mattress you first have to have an ulcer. I want to prevent one that is coming, not wait until one develops. That is is the wonderful world of Medicare requirements.

Thanks for all your replies. I am going to get in touch with her podiatrist this week as he is due for a home visit.