Air Tites For UK 50 Pence Coins - Recommendation

I would get the 28 might not fit in the 27 if both are true to size.

I acquired a 1937 George V1 gold proof £5 a few years ago.
Diameter 37mm plain edge. (technically a pattern, due to the plain edge.)

It now protected in a hard plastic acrylic screw positive seal capsule, in a display box originally intended for a silver proof British Crown. The only problem is that the capsule was for a coin of 39mm diameter, not 37mm.

Problem solved:
Around the edge of the £5 gold coin I cut a thin sliver of spongy styrene foam recovered from a food package container, and placed this as an insert around the edge of the gold coin. This arrangement protects the edge of the gold coin, which is notorius for being lightly scratched, when it was taken out of it's original proof set box. The styrene foam provides best possible protection for what is an extremely delicate edge.

My 1937 £5 gold proof remains at pristine perfect FDC* (equivalent to Sheldon PF 70)
It remains perfectly protected and displayed in it's own presentation box.
The box and capsule albeit originally intended for another coin.

* FDC = Fleur De Coin (a French term used for pristine coins in the British grading system)

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Jersey also used a foam insert to house their square silver proof and gold proof £1 coins of 1981, that were housed in hard acrylic screw seal crown sized capsules.