The two top bolt holes for the tabernacle were drilled from the outside, using a drill guide to make sure they were perpendicular, like this.
The two bottom holes were then drilled from the inside, as here.
I then loose-fitted the tabernacle, using the centre line to make sure it was in the right place, like this.
The bolt holes were then marked in on the back of tabernacle from inside the cabin.
The tabernacle was disassembled and the bolt holes drilled in the back piece on the pillar drill, as here.
The mast occupies the whole interior of the tabernacle so the bolts have to be countersunk. I did this with a Forstner bit.
The back piece was then bolted in place to check fit.
That looked OK so I reassembled the tabernacle and made a test fit, like this.
Large washers are used the inside the cabin, to spread the load on the backing plate. As in this pic.