Perhaps only 1 in 5 quoted goes ahead, people seem to think that material prices go up proportionally with size, but in reality the choice of certain oversize materials is limited and the costs considerably higher than the stock or standard sizes of 32" x 44" (some suppliers standard size is 32" x 40" and 32" x 47") and the jumbo size of 44" x 64" (some jumbo boards are 40" x 60"). Other factors that need to be considered are the extra labour involved, the equipment needed, and workshop space required to handle the assembly and fitting of large frames. The images below are just to give an idea of some methods that can be used.
There are some other methods that don't use a mount, for example a wide flat wooden slip that has a bevel edge can be used to replicate a mount, this can be painted or gessoed and lined with mountboard, so that the artwork is not in direct contact with the wood. An aperture could even be cut out of a sheet of MDF which can then be painted. Another option is that the artwork, print, or poster can be float mounted on to a backing, with a spacer to keep it away from the glazing.
Sheets of Foamboard, Correx, Gatorfoam, Kapa, and Tycore board can be used for backing and support, these are rigid, pH neutral, and lightweight. These types of board are all preferable to MDF or plywood which are heavy, prone to bowing, and contain impurities which can be harmful to art. MDF can be used but it is better to also use cross braces to make the backing more rigid.
For the glazing, acrylic in 4mm to 6mm offers the most practical solution for large sizes, it is lighter than glass and virtually unbreakable. Laminated glass can also be used but it is very heavy. Both acrylic and laminated glazings are available with anti-reflective and UV filter properties, these are known as 'museum glazing', and are used for the highest specification framing projects.
The frame itself of course needs to be suitable regarding strength, width, and depth, although even relatively narrow frames can be used if required when combined with a sub-frame, using pocket hole screws to secure the sub-frame to the frame. This example shows a bevel cleat which can be used to hang the frame to a matching section of timber fixed to the wall.
Where cost is an issue and conservation is not then large posters can be framed directly against the acrylic glazing, as acrylic is much less prone to developing condensation on the inside of the glazing when compared to glass.