If the tabernacle frames were open gridwork, what enclosed the back of the holy of holies?

This text is from a letter written by David Gooding in 1991.

Your question refers to the frames of the tabernacle. You ask, 'If they were only open gridwork, what enclosed the back of the holy of holies?' Your letter then adds, 'the tabernacle curtains did not, as far as I understand'.

But, in actual fact, the tabernacle curtains did cover the back end of the holy of holies, as I think you will see if you look again at the directions in Exodus 26:1–6, 33. The length of each tabernacle curtain was ten cubits, and they were placed over the tabernacle, starting on one side and extending over the roof and down the other side. Five of the ten curtains were also joined together along their long edges to form one group, and similarly the other five were joined to form one group. They were then joined together with fifty clasps of gold, again down their long sides.

Now, the tabernacle curtains, being each four cubits broad, when five of them were stitched together in one group, that group would be twenty cubits broad. The first group was then placed over the tabernacle with its long edge at the very front, and would thus extend twenty cubits down the length of the tabernacle. That brought this group to the point at which the four pillars upheld the veil of the tabernacle, for the Holy Place was twenty cubits deep.

With the first group reaching then to the veil, that would leave the ten cubits of the Most Holy Place to be covered in order to reach the back wall of the tabernacle, and then the ten cubits high of the tabernacle back wall. That would amount in all to a further length of twenty cubits; and that twenty cubits was covered by the second group of tabernacle curtains, which were each four cubits broad; being five of them, they would be twenty cubits broad when sewn together.