Here I've attached the skids ~ pointing in the right direction ~ using 40 x 6 mm bolts that are countersunk so as not to catch whichever surfaces we might be alighting on. Note too (by standing on the upright structure if necessary) that we're weak in the side-load department until such time as we fit shear webs as shown, in the form of the sort of brackets used for building kitchen cabinets. They cost just pennies, and got from the legendary Garners' in Skelmersdale, ironmongers to the mining trade since 1878.
The brackets also include pre-drilled holes matching the size of my pop-rivets, which is a marriage made in heaven if ever there was one.
