[Chris is me]

Shaker's working on a pretty similar prototype (but even shorter!). Insert your favorite variant of "great minds think alike" here.

I too think the lightening is a bit of a stretch. Taking material out of the sides (especially the sides perpendicular to forces in a box tubing scenario) weakens it a LOT. Considering you can cut a bunch of weight with square bearing blocks I would do that.

If you can think of a way to tension the blocks without using sliding bearing blocks, you can use unpocketed 1/8" tube to support your bearings. A lot of teams ran cantilevered wheels off of Toughbox Nanos last year which are made of the same stuff. This could save you a little weight and a lot of strength considering how aggressive your pocketing is around the bearing blocks.

However, unpocketed 1/8" tube for a whole rail is pretty hefty, so you may want to use 1/16"... but 1/16" of material is not a lot to hold a bearing with!