Bev, I was mulling over how to draw a little illustration of a way to
make your mat cover and it occurs to me now that a good thing to do in
this case would be to bind the whole thing, as that way when you get the
inevitable edge wear you can just replace the binding, whereas if you
stitch the perimeter seams and turn them (in a traditional straight
seam), the wear will occur on the outside of the seam, and in order to
take a new seam (to cut off the worn part) you would have to reduce the
sides too much.
So, here's what I think I'd do (gave up on the illus, not my talent),
using made-up measurements, of course. I'll say the finished size is
about 3X6', and for the sake of this discussion the open end is the
short end. I've also allowed wide seam allowances that might not suit
you, obviously all that's adjustable to suit.
Cut a front 38X80", and a back piece 38X90". Bind short edge of
each piece. Line up the unbound edges with wrong sides together and
stitch around them twice, trim near the seam, then bind that seam, on
the outside, on all three sides. Bag, roll, and insert the foam all the
way to the stitched end of the cover 'pocket' (keep the distal end in
place in the pocket with binder clamps or something, Ron's hands maybe),
and smooth it all out. Tuck the longer edge of the back piece over the
top of the foam so that it's between the upper cover and the foam. Pin
that in place, and take the loose end of the top cover to the back, pin
it down smoothly over all. Then mark the back piece for your Velcro
placement, unpin the top flap and mark Velcro placement on that. I'd
put the hook end of the Velcro on the back. Take all the pins out, and
may have to remove the foam (depends on your working space, manual
dexterity, and machine throat space) and stitch your Velcro strips
down. Might want to add a little pocket somewhere (I put pockets on
everything now that there's nobody here to put frogs and dead moles in
them.) and you're done.
That way it's pretty easy to do, and the part that will wear first,
edges, is readily replaceable.