I have that in my yard and it works well. Unless you want to grow a lot of long carrots, potatoes, or other root vegetables, 6" of soil is sufficient. A bunch of different companies make raised beds like that if you want to use a kit. I bought a kit made by Gronomics from an online retailer. The kit I bought can accommodate about 9" of soil so I left a bit of empty space at the top of it.

When I was in Ireland, I saw a veggie garden outside a restaurant. The wood box raised beds were up on cinder blocks and were about 18" deep.

You can get raised beds and planters specially aimed at people who want to avoid bending down to do their gardening. This raised planter table, for instance, comes from a range of Accessible Gardening products designed for people with limited mobility. It's just under 3ft high and has a planting depth of about 14", so you wouldn't have to limit yourself to small plants.

Accessible Gardening · More Info

Unfortunately this particular model is only available in the UK, but it might give your husband a starting point for his design. As for drainage, I think a cloth liner is a great idea and will help protect the wood. But make sure you use tanalized (pressure-treated) timber, or the planter will soon rot.

At Costco I saw some raised beds in different formats, and they have some of the listed online. If you are making your own, they might give you some ideas. I really like the shelf ones.

https://www.costco.com/raised-garden-beds.html

Here are lots of plans, ideas and sources. All free - just download and compare types costs and you will be sure to find one to fit your needs.

http://accessiblegardens.org/plans-for-raised-garden-beds/