Joe makes this easy and fun. If you respect your time, buy his plan.
I built the XL version. It was fairly easy to fill in the gaps from the regular diy plans that are more robust, but my recommendation, as is the recommendation in the directions, is to take your own measurements as you move along as small adjustments are needed at times. Some of this was caused by me in having the big box stores pick my materials and have them ready curb side, so I ended up using some warped and bowed wood that ended up leaving things out of square. An obvious recommendation, but pick straight lumber. Also, go slow with the aluminum wrap and tape so it's as smooth as possible and check for "square" early and often. If you are going to use wheels put on at the first opportunity. Based on my errors I decided to not go the "disassemble" route and this is a permanent box now. It's not coming apart and it's is very, very heavy.
It is beautiful when done however. It is very hot and I am now looking for sauna pads and towels as the pool of sweat from a 15 minute sauna is legit.
Bought this plan and the material all within 24hours ... and then had everything built over a day and a half on the weekend. Incredibly easy to follow & the videos remove literally every question you could have. I highly recommend and this thing gets HOT!
I had been looking at getting a Sauna for 5 years at lease. Was constantly turned off by the price, size and then having to get it wired up. Found this one and decided to give it a go. It does work as advertised.
If I was to build this again, I think I would change one thing. Length and width are fine for me but I think I would add two feet in height. I can see it being difficult to pull myself up as I get older. I would like it if I could have a small stool to sit on instead of the floor.