You are absolutely on the right track. Kapton is the way to go.
I’ve got a lot of experience with this and purchased the following PEO build plate. Unfortunately, it was available for only a limited time but you might find it elsewhere on Aliexpress. The key word search is “Specular” which gives the mirror finish you are looking for.
Here’s how “glassy” the plate looks in my printer with the LED riser lights turn up full.
That image was a test I did showing that one can use a sparkly tape to achieve a sparkly surface. I printed the disk over the two surfaces on the PEO plate. Here’s what the final product looks like and compared to one printed on a PEI plate. Click to zoom in to see that the filament structures are very visible on a specula surface. The filament was PLA.
Save your money though because after I purchased that plate, I then had a happy accident with Kapton tape and figured out that this was a better deal.
As you already surmised, you can get the same effect with just plane tape. Kapton of course is best. I found this nearly full size sheet of Kapton for cheap. I used it on my smooth plates and it performs identically to the specular PEO plate I have if not better. By better I mean it released easier while still allowing for good 1st layer adhesion.
I use that plate multiple times a week and I’m still on the first of 10 sheets, although it is starting to show it’s age with all the nicks I have in it from being too aggressive with the plastic scraper.
Here’ what my build plate looks like with the same sheet I’ve been using since last July. I overlaid it onto my engineering plate but truth be told, I think it would work on any smooth plate. No worse for wear but I have 9 sheets left. Although it is not nearly as mirror-like as the PEO plate, one cannot tell the difference once the filament is laid down.
Their is a trick to applying these sheets well. Go up onto YouTube and do a search on “DIY Window Tinting”. Just follow the same technique. I used Windex as my float agent and then used a rubber squeegee to smooth out the bubbles and then finally a credit card for the more stubborn ones. Then I monitored it while it set up to make sure no more bubbles came up.
Here’s one guy who shows a demonstration of applying that filament using dishwashing soap and water as the float agent. I used Windex solely because if that’s what professional car tinters use, it must be good enough to withstand the heat of a car and to be honest, I found it worked very well. You need that layer of liquid to make it easier to apply and lift again as you squeeze the bubble out but of your really skilled you probably can just apply it directly, the Gizmodorks film has an adhesive backing if I recall.
I tried Garolite too but it did not produce nearly as nice a surface as did the Kapton tape. Here’s a link to that discussion with images of what garolite surface looked like. Super simple DIY garolite G10 build plate for X1C - #12 by Olias