Best way to temporarily block off heater hoses?

Ever since I've owned my Valiant the windows seem to fog up or get filmy overnight. The previous owner had the same problem but didn't even try to figure it out.

It's more than just dirty windows, more like an oily film, so the first thing I think is heater core, however it isn't steaming, no anti-freeze smell and no coolant loss.

I figure the best way to see if that's it is to block off the heater core and drive it like that for a week or two.

Normally I would just loop the heater hose back to the block, but on a slant it's hard to do without a lot of hose and that looks awful, but you usually end up with a pinched hose if you use the stock length.

I've seen people put bolts in hoses and clamp them there in the past; is this a safe thing to do? How much pressure goes thru the heater system?

If the core ends up being the problem I'll likely drive it blocked off for a few weeks before I can afford or have time to change it.