The t-slots in the back and side of the Misumi extrusion are the same as in the 20x20 and makerslide.
I used a center punch and center drill to start the hole to try and ensure good placement, that might have been overkill. Then I used a 10mm endmill to clear out enough space in the slot for the bolt head. I drilled a 7/32' hole through the 20x20 at positions along the length that matched the position of the small slots in the back of the Misumi extrusion (35mm, 215mm, 285mm and 465mm). So I drilled the 20x20 extrusions instead.
They came with a slight warp to them, but that seems to go away when they are bolted down to a 20x20 recommended drilling through the Misumi extrusion to bolt it to the 20x20, but unfortunately, my mini milling, drilling, lathe setup was not up to the task of drilling something that big. I used two of the Misumi 250mm extrusions cut at the supplier to 500mm lengths.
Hopefully this might help anyone considering using the same extrusions. So I am making good progress on upgrading my 500m X carve with an aluminum t slot bed.
