TX
I didn't have any velcro kicking around when I did this, plus, I don't think there was enough surface area to use velcro anyway where I have it stuck to, maybe on the top and side would work. I used some 3M doublesided adhesive on the top only. Considering the amount of times that I would remove it, I'll just peel some more off the roll if I have to.
I didn't document what I was doing, just measuring 50x then cutting, then measuring, then cutting... took quite some time.
I would recommend that you use a dremel tool to do the cuts, along with drilling the corners where you want the cuts to end.
If you fracture the plastic, which is really easy to do, drill a hole at the end of the crack to stress relieve it so it can't crack any further. I only did this one time when trying to get things to fit, oh well.
I could make up a drawing in CAD with some precision measurements, would make it easy for anyone to do. Of course everone else would have one that looks much better than my cut and try method

If I get around to it, I can pull it out and get the dimensions and some photos of the install. If someone sends me an extra ashtray they happen to have I'll get it done ASAP and photo the install.