For parts at right angles these magnetic pieces do a good job

Tools and Storage - York Model Rail

In the past I've often used elastic bands to hold a roof in place, sometimes a problem with the narrow elastic distorting parts if too tight, whereas if not tight enough parts may not be held in place.

These hook and loop strips are wider so little chance of distortion especially as you determine how tight they are.

Mudder Multipurpose Adjustable Hook and Loop Securing Straps and Fastening Cable Ties, 16 Pack: Amazon.co.uk: DIY & Tools