Filament choice for coasters
Even if you pour in boiling water in a cup, the outside of the cup will have a lower temperature. When resting on a coaster, usually a small part of the cup actually touches the coaster. Also, the design of the coaster could influence the heat transfer, a more open structure of the coaster may be beneficial. Some people print coasters in PLA although the glass temperature (temperature at which the plastic becomes soft, this is usually the temperature of your bed when you print the material) may be lower than than the temperature of the cup, the filament will weaken (for PLA at about 50&nbsp;&deg;C), but not melt , melting of PLA happens at a higher temperature (for PLA above 150&nbsp;&deg;C) than your mug can get (unless you pour liquid metal in it). It should therefore be doable in PLA, I have printed a PLA coaster that has very small ridges (about 1&nbsp;mm) of an embossed image and placed a cup with boiling water on it, to find that the coaster is able to withstand the temperature of the cup without deforming (the ridges do not fail or deform).

To elaborate on the filament materials other than PLA; there are many filament types that have higher glass temperatures, but are still very printable. Various types of co-polyesters exist, like PETG, that have a higher glass temperature (>&nbsp;85&nbsp;&deg;C), are a substitute for ABS and still very well printable. Nylon is also a material that can be used, there are brands that have low warp nylon.

Note that there are a few options to print ABS while you have a non-enclosed printer, you could 


fabricate some cardboard pieces for a temporary enclosure, 
place the printer in an non-draft room or 
print the skirt height at the height of the print part.


Note that coasters have a relative low profile height, it should not be that big of a problem to print ABS coasters on an open printer.