So, I have a tutorial on how I make my movement trays. When I was getting that link for a post I made on a forum, I noticed that I'd done it back in 2010. I'd also said I'd show how I magnetize them later... Well, I think 4 years is long enough! So this is how I do it.
Once the tray is made, I then cut to size the sheets I use to line the trays. For my trays I use a product called Flex-Steel (I think? I cant find the labels anymore lol). It is not magnetic itself, but is ferrous enough that anything magnetic will stick to it. Its self-adhesive peel and stick so makes the process a lot easier.
Cut it to the inner size of the tray.
I like to align it to the front rail of the tray and then slowly work it down so that there are no air bubbles. Use another piece of plastic to help squeeze the sheet to the floor of the tray to make sure it has adhered thoroughly.
For the magnetic bases, I use a .020 inch thickness magnetic sheet with peel and stick backing. For 20mm bases I cut strips 21mm wide
I then cut those strips into 21mm squares
Why 21mm by 21mm? This will make aligning the base onto the magnetic square a lot simpler as it gives you more room to play with. GW bases aren't exactly 20mm square, and can vary up or down in their size... Having that extra 1mm makes it a lot easier to get it stuck on.
Now, you need to prep the bases themselves. If you are using slottabases, then you can skip this part. However, if you are using the normal flat top base, you need to add a spacer for the magnetic sheet to grip to in the middle. This way they stay stuck to each other far longer than without. I use just clippings of extra plastic, which needs to be at least 2mm thick. Less than 2mm and there wont be good adhesion. You don't want to use to thick or the bases wont sit flush. Glue the plastic spacer into the bottom of the base like so. I use a plastic weld type glue, but you can use superglue if you want. I prefer the welding as it will stay together longer.
When attaching the magnetic sheet, I like to strip the backing off and then lay the square on the table sticky side up. I then align the miniatures base and slowly lower it until it touches. I will then pick it up, pressing firmly in the middle to adhere to the spacer, then I will run my fingers around the edge to press it all together. I'll then sit it on the table and press firmly down along the base edge to make sure the seal is good.
Then trim with your hobby knife. I like to go at a slight angle so there is a bevel like /. This way there is less chance of the magnetic sheet catching at the join and ripping away from the base.
Just finish up your unit, and you are done.
As you can see, it holds pretty well.
Now, this works great for plastics. Metal minis you would have to use Magnetic sheeting on both the bases and the trays themselves. I'd recommend using the thicker .030 inch on the trays as this will improve the magnetic bond as well. I also recommend that you do not touch the sticky backing with your fingers... skin contact really weakens the stickiness of the backing...
That is pretty much it. I hope this was informative and inspirational. If you have any other questions on the process, or if you have any suggestions, feel free to post them. Thanks for looking.