Tape is hateful. I like glue in partiular Vittoria Mastik one Other tubular cements are just not as good - not strong enough or more messy.
Paint a good layer of mastik one on the rim and tyre and leave overnight (or for a few hours in a warm room till it hardens). Then mount the tyre, inflate to 20 or 30 psi and centre it on the rim. Inflate to max pressue or 100 psi which ever is lower and quite quickly you will have a very firm bond. Its a contact adhesive, so it bonds on contact.
This method differ from Vittoria's instruction but does not leave mess on you or the tyre sidewalls. The glue takes more than 5 minutes to dry in England.
Apply a layer to your spare tub which whould be stowed under the saddle with a toeclip strap or tyre holder. At the roadside remove your punctured tub and fit your preglued spare. The bond will be strong and this is the beauty of Mastik One Glue and it makes tubular tyres useable for normal riding.
Tub gluing is a ritual. It is the whole point of tubulars. Make a cup of tea and use the best and try not to get it on your clothes.
If you buy 30g tubes well that enough only for one rim so you might as well buy a 250g tin. Dont skimp on the glue your life quite litterally depends on it.