I just finished mine, the bikes not running yet so not sure what it'll be like on a trip but it's nice and wide and seems comy making vroom vroom noises in the garage.
I used an aluminium stop sign with a cross brace across the seat rails to take the weight. I put 1/2" closed cell gym mat and 1" high density open cell foam. I ended up getting the local uphosterer to cover it. On the bottom he sewed a sleeve all the way around and put that sprung wire with a plastic type coating used on light weight curtains etc. I haven't seen that done before but looks to spread the tension evenly all the way around. Don't know what it'll look like with the tank till I finish with the paint tomorrow and let it dry.
Here's a couple of pic's seems a fairly cheap way to go.
Good luck with yours