I helped put a couple of floors in a friends car. Even with the real thing its hard work at best and that was with a lift and air riveter. I would not do one again.
Sometimes one just has to change ones mind ...
Having fitted lowered floors to both side of my car (on my own!) I agree with Alan. The original floors are 1.5mm sheet, the lowered floors are formed from 3mm sheet and substantially heavier and stiffer, I had to do a lot of adjusting/trimming to get a good fit, then removed them both to be powder coated and refitted. If the floors you have made are even less accurate than the Arch/Caterham originals it'll be a thankless task.
Also note that on the Arch lowered floors, unlike the original floors the outside edge is vertical not horizontal.
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Hobbsy, you will need loads of steel rivets and an air riveter, and having built a few 7s I know my way around spannering the cars, but it was a no brainer to get Arch to do mine, when you have done yours, you'll know why.
In a moment of madness or boredom I started to fabricate my own lowered floors based on the dimensions above - I still have them and although would fit with some further fabrication I would not be happy with appearance. Lesson learnt - some jobs are best to leave to the experts!
Arch can supply and the fit is not too difficult if travel an issue.
You could always go about this the other way. If you're approaching a time where you won't need the car for a while remove the current floor and measure the opening in the chassis. I'll get round to making a lowered passenger floor at some point and this is probably what I'll do. You know it will fit then.